The invention relates to a system and method for monitoring the composition of a magnetically permeable material which may be a magnetically permeable fluid. More particularly the invention relates to a system and method for monitoring the composition of a magnetically permeable material by sensing the inductance of the material and mass flow rate of the material through the system. The sensed values of inductance and mass flow rate may be used separately or in combination to determine the actual volume fraction of particles in the material.
Magnetically permeable materials such as magnetorheological (MR) fluid is comprised of magnetizable particles suspended in a liquid, which might be oil, water or silicone for example. Before the introduction of a field, such MR fluids are typically free flowing. With the introduction of a field, such as a magnetic field, the particles form a dense network and the apparent viscosity of the fluid changes making the fluid thick. The degree of change is proportional to the magnitude of the applied magnetic field.
Magnetically permeable materials are frequently used to control the displacement of a moving member such as a piston in a damper or a rotating component in a braking device. Such materials are specifically designed to suit the associated application for the fluid. The specifically designed materials must have the requisite volume fraction of magnetizable particles and must include the requisite type of iron in order to be able to provide the requisite resistive forces that produce the requisite motion control of the associated device.
The magnetically permeable material, such as MR fluid, is manufactured by mixing the iron particles and carrier fluid. The iron particles and carrier fluid must be mixed at the precise ratios to produce a volume of fluid with the required magnetic properties. As the fluid is mixed, the fluid is tested by a technician to ensure that the required consistent fluid composition is achieved and maintained. The prior art method for testing the magnetically permeable material is comprised of using a density cup manufactured by Paul Gardner Group of Pompano Beach, Fla. to test discrete volumes of the material. In practice, a volume of collected in the density cup. Once the density cup is filled with the required volume of fluid, the precise collected volume of the viscous fluid is determined using a complex method that involves diluting and mixing the sample of the fluid with an acceptable diluting agent which frees the viscous material sample of entrapped air. The diluted sample is then weighed and from the weight and fluid volume the volume fraction of magnetizable particles may be determined.
There are a number of problems associated with the method for testing the manufactured magnetically permeable material using a density cup. First, the material is tested intermittently. Because the material is tested discretely rather than continuously, only a small portion of the total volume of material is sampled. As a result, the mixed material composition may be inconsistent and comprised of variable and undesirable volume fractions of magnetizable material. Additionally, the density cup sampling procedure is highly specialized and requires specific, extensive training in order to develop the expertise needed to regularly obtain accurate measurements. Therefore, the density cup testing technique is prone to operator error.
The foregoing illustrates limitations known to exist in present systems and methods. Thus, it is apparent that it would be advantageous to provide an alternative system and method for monitoring the fluid composition to maintain a desired volume fraction and type of magnetizable particles where the volume fraction of the material is measured continuously as the material is mixed and is not prone to operator error. Accordingly, a suitable alternative method and system is provided including features more fully disclosed hereinafter.
In one aspect of the present invention this is accomplished by a system and method for monitoring the composition of a magnetically permeable material, where the system comprises a first container containing a volume of magnetically permeable material; a flow path for flowing the magnetically permeable material out of the container; a sensor for determining the inductance of the magnetically permeable material; and a sensor for determining the density of the magnetically permeable material. The sensors for determining the density and inductance of the magnetically permeable material are flow connected to the flow path.
More specifically, the invention uses an inductance sensor that is sensitive to the magnetic permeability of a magnetorheological fluid to measure or monitor the volume fraction of magnetic particles in the magnetically permeable fluid. The sensing inductor is generally hollow with a coil surrounding the hollow body. The hollow member may be annular or tubular. An inductance meter or bridge circuit is used to measure the inductance of the coil. The measured inductance is proportional to the magnetic permeability of the material contained in the coil. In this way, the desired volume fraction in the material is maintained. The density of the material is monitored by a mass flow meter which may be a Coriolis type flow meter for example. By measuring the material density a technician can confirm that the required type of magnetic particles were mixed with the carrier fluid.
The values of inductance and mass flow rate may be used separately or in combination to determine the particle volume fraction.
The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures.